The present invention relates to a method for controlling completeness of removal of a photoresist layer in openings, in particular in round, substantially closed openings of a photoresist mask.
In the field of control of results of lithographic processes, a control of completeness of development of a photoresist layer is not yet solved, as specified for example in A. Starikov, K. W. Tobin, “EM review and discrete inspection of pptically invisible defects in a production environment”, Proc. SPIE Vol. 4692, pp. 162–167, 2002; and H. Nishiyama, M. Nozoe, K. Aramaki, O. Watanabe, and Y. Ikeda, “Open-Contact-Failure Detection of via Holes by Using Voltage Contrast”, Proceeding of SPIE, 4344, pp. 12–21, 2001. Incomplete removal of the photoresist in the openings or “windows” of a photoresist mask leads to rejects in the manufacture of integrated circuits, to reduction of yield of articles, and therefore to substantial worsening of efficiency of operation of facilities which produce microelectronic devices. This problem is resolved nowadays by accurate selection of modes of exposition and development of photoresist during adjustment of technological processes of lithography. During the adjustment, in order to control the completeness of development, various methods for determination of a residual layer of photoresist on a bottom part of the “window” of the photoresist mask are utilized. Mainly, these methods are used in condition “out of fab”, they are not related to the group of so-called “non destructive” methods, and they are realized in R&D departments outside of the main technological process production of microelectronic devices. From economical point of view, the control of completeness of development is frequently performed not on the working wafers, but instead on specially made test modules and wafers, which makes difficult to use the thusly made conclusions for real products.
The above mentioned difficulties are especially pronounced when the control for completeness of development of a photoresist in the “contact windows” has to be performed in the windows which are formed as round openings with a small size in the photoresist layer. The reason is the differences in kinetics of development of photoresist in narrow and broad “slots” or in broader “windows” of the photoresist mask. Theoretically it is understood and experimental it is proven that the conditions for complete removal of the photoresist in the narrow round openings do not correspond to those for the system of strips of the same size, or for larger round and other windows in the photoresist layer. It is therefore necessary to develop a method for controlling the completeness of removal of photoresist in contact windows of small sizes on the working plates or wafers in condition of the mass production of the microelectronic articles. It should be mentioned that the incompletely removed photoresist layer on the bottom part of the opening has an island structure. This is caused by many reasons, and the main reasons are micro-inhomogenuity of the properties of the photoresist, and in particular of the kinetics of its development in narrow openings.
An attempt to solve this problem is described in S. Majoni, I. Englard, “CD-SEM-Based Metrology for Contact Lithographcy and Etch Control”, Semiconductor International, Apr. 1, 2003. In this attempt a correlation between the completeness of removal of photoresist in the “contact window” and the shape of inclination (side wall) of this window was determined. A greater completeness of removal of photoresist allegedly corresponds to the steeper inclines. A quantitative criterium (profile grade-PG) is presented which allows classification of all possible cases occurring in practice. In accordance with this reference, the case 0<PG<1.4 corresponds to the bottom of the “window” completely covered by resist (closed). If 1.4<PG<1.7, then the resist layer in the “window” is removed incompletely (semi-opened). The value 1.7<PG<5 corresponds to the complete removal of the photoresist layer (fully opened).
In addition to arbitrary selection of the numeric borders and conditional nature of the determined “closed”, “semi-opened” and “fully opened” situations, the proposed method suffers from the fact that a method of calculation of the criteria PG is not disclosed. The calculations of the value PG on a picture of the cross-section of the wafer is not advisable, since it presupposes the destruction of the expensive object of control. The authors recommend to use a scan electron microscope, and in particular “the secondary electron intensity waveform to determine the profile of a contact”. However, it is well known that the secondary electron intensity waveform in the electron microscope never coincides with a geometry of the object, and there is no fully determined connection between these characteristics. The secondary electron intensity waveform depends significantly not only on the geometry of the object, but also on its properties (composition), and also on the characteristics of the used electron microscope: its resolution, accelerating voltage, type of detector of secondary electrons, value of a working distance, level of noise in a video signal, etc. Correspondingly, when the object of control is changed and the type of the microscope and parameters of its operation are changed, inevitably the values PG calculated from the analysis of the shape of the video signal will change as well. Therefore, the problematic correlation between the structure of the side wall and the completeness of removal of the resist layer on the bottom of the “contact windows” becomes even less accurate, in the case if as it is recommended in this reference, the key value of the parameter PG is calculated from the shape of the video signal. Finally, it seems unnatural and therefore inefficient when a conclusion about the properties of one object, namely the bottom of the opening, is made based on the results of the other object, namely the wall of this opening.
U.S. Pat. No. 6,303,931, proposes a method which is substantially similar to the method presented immediately herein above. An approximation is used of the video signal obtained in the experiment with a certain analytical curve, or a power function of the type:       y    ⁡          (      x      )        =                    (                              y            2                    -                      y            1                          )            ⁢                                                                              x                -                                  x                  0                                                                              x                  1                                -                                  x                  0                                                                          n                                                                                                  x                  2                                -                                  x                  0                                                                              x                  1                                -                                  x                  0                                                                          n                      +          y      1      
wherein the function y(x) reflects the dependency of the value of the video signal (y) from the coordinate x along the line of scanning, x0, is a position of the central minimum on the video signal, x1 and x2 are points on the left and right inclines of the video signal selected, for example as the points where a derivative of the video signal is maximal, y1 and y2 are values of the video signal in the points x1 and x2 correspondingly. The parameter n which has a key value is determined during the procedure of approximation of a set of values of the video signal with the above mentioned analytical curve.
This method also has substantial disadvantages which in some cases will lead to significant errors, or to the situation that it does not work at all. In a typical case when the central minimum (coordinate x0) is located at equal distances from the selected points x1 and x2, in the denominator of the fraction under the sign of the module exactly one is obtained, and the whole denominator will be equal to zero, and therefore it will become impossible to calculate the approximated function and the key value of the parameter n. In another example, if in the selected points on the inclines x1 and x2 the values of the video signal y1 and y2 are equal, then the first multiplier before the fraction in the formula will be equal to zero. Therefore the whole first adding element will become equal to zero, and the dependency y from x will no longer exist. This also will make impossible the determination of the value n. Also, another example can be presented. During evaluation of possibilities and limitations of the method it has to be taken into consideration that the video signal always have noises. It is shown in practice that the ratio signal/noise (S/N) is small and seldom exceeds the value 10. Since the noise of video signal has a statistic nature, it can be said that the value of the video signal measured in the experiment in any point of measurement is a random value, and in accordance with the statistic laws, with the probability 68% it does not go beyond the interval from SAVE−N to SAVE+N, wherein N is a mean square value of the noise, and SAVE is a mean value of the signal averaged over many realizations. This is applicable also to the points x, and x2. As a result, the difference y2−y1 due to random reasons can also assume the value from zero to 2N. Since in the above mentioned formula this difference is a comultiplier, the calculation of the key parameter n becomes a very unstable procedure. If a signal from the same object is determined twice, then the value y2 and y1 in the above mentioned points will be different (due to the contribution of statistic noises, which inevitably will lead to different values of the parameter n).
Finally, the authors ignore the dependency of the steepness of the profile in the video signal, and therefore of the parameter n from the diameter of the beam of primary electrons which scan the sample, and also the dependency of the shape of the profile from the ratio of the diameter of the openings to its depth. These ratios which are not taken into consideration, lead in practice to an additional ambiguity during the calculation of n.